1. Field of the Invention
The present invention is directed to a method for acquiring (identifying) eddy currents that are caused by switched magnetic field gradients in a magnetic resonance apparatus and that contain cross-terms.
2. Description of the Prior Art
A method for the compensation of eddy currents caused by gradients in a magnetic resonance apparatus is disclosed in German OS 43 13 392. The gradients serve the purpose of generating a magnetic field gradient. This magnetic field gradient is needed in order to generate a spatial resolution of the nuclear magnetic resonance signals in magnetic resonance tomography. To this end, a uniform, static basic field on the order of magnitude of 1 Tesla has such a magnetic field gradient superimposed on it. For a spatial resolution in three dimensions, magnetic field gradients must be generated respectively in three directions that preferably reside perpendicularly on one another. A substantially constant magnetic gradient Gy in the y-direction is generated by gradient coils within a spherical examination volume. The gradient coils for the magnetic field gradient in the x-direction are constructed identically to the gradient coils for the magnetic field gradient in the y-direction and are merely rotated by 90xc2x0 in azimuthal direction. The gradient coils for the magnetic field gradient in the z-direction are annular and are arranged symmetrically relative to the mid-point of the examination volume. Two individual coils respectively have current flowing therein in opposite directions and thereby generate a magnetic field gradient in the z-direction.
The required magnetic field gradients must exhibit steep leading and trailing edges and must be as constant as possible during the on duration. Due to the steep leading and trailing edges, however, eddy currents are induced in metallic parts of the nuclear magnetic resonance apparatus, particularly in the inside tube of the cryostat surrounding the examination space, these eddy currents in turn generate magnetic fields that are directed opposite the magnetic field gradient. This leads to a rounding of the corners of the desired square-wave pulses and to a parasitic magnetic field that decays after the de-activation of the magnetic field gradients.
The curve of the magnetic fields B(t) caused by the eddy currents can be represented as follows:
B(t)=B0(t)+Gx(t)x+Gy(t)y+Gz(t)z+0(t, x2, y2, z2),
wherein B0(t) is the location-independent term of the 0th order, Gx, Gy, Gz are the terms of the first order and 0(t, x2, y2,z2) is the term of the second order.
The terms Gx, Gy, Gz of the first order thereby dominate. Only these can be compensated in the known eddy current compensation techniques and must be exactly measured. The terms of a higher order, particularly of the third order, can not be compensated with the known methods and are only of significance for greater distances from the center. The location-independent term of the first order B0(t) is generally small and can arise, for example, from the asymmetrical arrangement of the gradient coil in the magnet or from other effects that disturb the symmetry. It is adequate for definition of the terms of the 0th and of the first order to measure the magnetic field arising from the eddy currents for each direction x, y, z at two points spatially separated in the respective direction.
In the method according to German OS 43 13 392, a spatially expansive phantom, as is employed in magnetic resonance tomography systems for other testing and setting purposes, is introduced into the examination space and is measured with a slice-selective MR method. An eddy current compensation without specific devices such as measuring probes and holders can be implemented and checked with the method according to German OS 43 13 392. The manipulation of the method is simple since the phantom need not be moved for the measurement. Further, it can be determined in a simple way as to whether additional inserts such as, for example, a surface coil lead to additional eddy currents. The method according to German OS 43 13 392, however, is not able to determine the terms of higher orders.
Further, European Application 0 228 056 discloses a method wherein the magnetic field curve is measured by the magnetic resonance signals induced in a specimen. Since the measurement of the magnetic field is required at at least two locations of the examination space, the specimen must be changed back and forth between two measuring positions for each measuring cycle. The specimen would have to be placed at many measuring positions for identifying eddy currents of a higher order, which would be extremely complicated.
German PS 43 25 031 and U.S. Pat. No. 4,910,460 teach measuring the eddy currents with an expansive phantom with slice-selective MR imaging. The methods disclosed therein only supply a spatial information but no time information about the eddy current distribution. These methods thus only allow the graphic presentation of the spatial eddy current distribution and are not suited for quantitative determination of the amplitudes and time constants of the eddy currents that however, would be necessary for compensation of the eddy currents.
Further, the article xe2x80x9cTemporal and Spatial Analysis of Fields Generated by Eddy Currents in Superconducting Magnets: Optimization of Corrections and Quantitative Characterization of Magnet/Gradient Systemsxe2x80x9d in the periodical xe2x80x9cMagnetic Resonance in Medicinexe2x80x9d, 20, pages 268 through 284 (1991) describes a method that is suitable for the quantitative determination of eddy currents of higher order. Similar to the methods according to German OS 43 25 031 and U.S. Pat. No. 4,910,460, eddy currents are graphically displayed by following a gradient pulse with an imaging sequence that generates a stimulated echo whose phase relation is proportional to the eddy currents. The time development of the eddy currents also can be identified by measuring at different time intervals following the gradient pulse. The disadvantage of this method is the long duration of the imaging sequence of approximately 5 seconds per Fourier line and the time interval following the gradient pulse, as a result of which a measuring time of at least ten minutes arises. Since the sequence must be repeated in a number of steps and for all three gradients, overall measuring times of far above an hour result. This is significantly too long for a method that is to be utilized in routine fashion for compensation.
An object of the present invention is to provide a method for acquiring eddy currents wherein at least the cross-terms of an eddy current can be identified in the selected slice within a short time.
This object is achieved in a method having the following, successive steps:
a) a spatially expansive phantom is introduced into the examination region of the nuclear magnetic resonance apparatus;
b) a measuring gradient pulse that exhibits a prescribable pulse width is activated;
c) after the deactivation of the measuring gradient pulse, at least two imaging sequence blocks following one another at the spacing (t1, t2, tn) are generated, with an at least two-dimensional, complex data set being generated from their imaging signals, and with the phase information contained therein being proportional to the magnetic field strength.
This method, in addition to supplying two-dimensional location information, also supplies time information about the distribution of the eddy currents within a selected slice. At least the cross-terms of the eddy currents thus can be reliably identified in a simple and fast way.
The simplest possibility of acquiring eddy currents is in an embodiment of the method wherein:
c) at least two gradient echo blocks serve as imaging sequence blocks, these being generated after the measuring gradient pulse is shut off and following one another at the spacing (t1, t2, tn) wherein, in each gradient echo block,
c1) a first, slice-selective RF pulse is emitted under the influence of a slice-selection gradient pulse, and
c2) the gradient echo generated as a result thereof and serving as the imaging signal is acquired during the decay time of the eddy current caused by the measuring gradient pulse, and
c3) a phase-coding gradient precedes each gradient echo, and
c4) the gradient echo is read out under a readout gradient in a direction perpendicular to the phase coding gradient, and
d) the steps b) and c) are repeated respectively with incremented phase code gradients.
In this embodiment, thus, at least two gradient echo blocks are provided as imaging sequence blocks. This embodiment, in addition to supplying two-dimensional location information, also supplies time information about the distribution of the eddy currents within a selected slice, so that at least the cross-terms of the eddy currents can be dependably identified simply and rapidly.
In a further embodiment of the method, the above steps b) and c) are repeated with at least one other position of the selected slice. In another embodiment, the above steps b) through d) are repeated with at least one different position of the selected slice. As a result, three-dimensional location information is obtained in these two embodiments of the inventive method in addition to the time information about the course of the eddy currents. In addition to the cross-terms, the terms of a higher order can be additionally identified on the basis of the now-available spatial and time information about the eddy current distribution. Since these embodiments of the inventive method allow a significantly faster acquisition of eddy currents that are caused by switched magnetic field gradients and contain the cross-terms and terms of a higher order, they are particularly suited for employment as routine procedures.
When, given a measurement of at least two selected slices, an acquisition of eddy currents of an order higher than the first order is desired without lengthening the measuring time, a further embodiment of the method can be used, wherein the gradient echo blocks of the various slices are interleaved in one another.
In further embodiments two-dimensional images can be calculated from the acquired eddy currents, the non-linear eddy current components can be calculated, and these non-linear eddy current parts can be compensated.